Hot roll fusers are known as one form of fuser to fix thermoplastic powder images onto carrier material, such as, for example, a sheet of paper. Such powder images are produced by the transfer step of the well known electrophotographic process, also known as xerography. In the hot roll type fuser, it is conventional to provide an internally heated hot roll whose outer surface is deformable. This roll cooperates with a clean and relatively cool backup roll to form a fusing nip through which the toned paper sheet passes, with the toner adjacent the hot roll. These two rolls are maintained together with a force which causes the backup roll to penetrate the deformable surface of the hot roll so as to form a footprint or impression whose area constitutes the fusing area or nip.
In some devices, the fusing nip is opened between copy sheets, while in others the nip remains closed during the inter-copy gap which separates sequentially fed copy sheets. With either type of device, and primarily with the latter type, there is a tendency for toner to accumulate on the backup roll, for example from direct contact with the hot roll, and/or form loose, air-carried toner within the xerographic printing system.
Prior printing systems have provided cleaning means to remove this toner and/or debris from the backup roll. For example, U.S. Pat. No. 3,794,417 discloses a scraper which includes a plurality of individual spring finger blades having sharp leading edges that scrape toner particles from the surface of the backup roll.
The present invention is an improvement to this generic type of scraping blade backup roll cleaner.
The present invention provides a cleaning blade having a sharp leading edge which is positioned in running engagement with the surface of the backup roll. The surface of this roll is preferably covered with a material having low surface energy, to thereby minimize the adhesive forces between the roll and the toner and/or debris. This material constitutes a destructible surface which must be protected from cleaning blade abrasion, and yet the cleaning blade must operate to clean the surface.
To accomplish this result, the cleaning blade of the present invention engages the backup roll's surface at a critical angle. The coefficient of friction of a clean backup roll surface is substantially less than the coefficient of friction of the same surface when it carries toner and/or debris. The sharp leading edge of the cleaning blade engages the roll at an acute angle, which is measured from the tangent to the roll at the point of engagement and in the direction of roll rotation, such that the resultant force component lies within the blade when engaging the clean surface, and lies within the acute angle, i.e. between the blade and the roll, when the blade engages a contamination filmed roll surface.
The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawing.